45th Lunar and Planetary Science Conference (2014) 2066.pdf

GLACIAL MODIFICATION OF EASTERN NEREIDUM MONTES, : OBSERVATIONS FROM SOUTHERN HEMISPHERE MAPPING OF LOBATE DEBRIS APRONS AND ICE-RICH FLOW FEATURES. F. C. Chuang, D. A. Crown, and D. C. Berman, Planetary Science Institute, 1700 E. Ft. Road, Suite 106, Tucson, AZ 85719 ([email protected]).

Introduction: Lobate debris aprons (LDA), While the large concentrations in eastern Hellas (CCF), and and northeast Argyre are generally in agreement with (LVF) are a suite of features present throughout the previous global surveys of ice-rich flow features northern and southern mid-latitudes of Mars [1-10]. [4,8,9,30], the features in Nereidum Montes have They are often associated with , regions received less attention than those in eastern Hellas. In in which the surface appears to have been "softened" [9], this area is consistent with a low density due to the viscous flow of ice-rich materials [11-14]. background distribution of ice-rich flow features LDAs in particular, exhibit similarities to several types poleward of 40° S. The likely difference between our of terrestrial cold-climate landforms [2,5,15] including study and [9] is the inclusion of large sheet-like lobate rock glaciers [16-17], interconnected valley glaciers features in the current study. With increasing surface [18-23], debris-covered glaciers, as well as protalus coverage of Mars by Context Camera (CTX) images, lobes and ramparts [5,15]. Modeling results from the we can now examine these features in detail. Shallow Radar instrument on Mars Reconnaissance Nereidum Montes CTX Observations: Nereidum Orbiter have supported a debris-covered glacier Montes is mountainous terrain along the northern interpretation for LDAs in which hundreds of meters- margin of the Argyre impact basin (300°-332° E, Fig. thick ice is sequestered below a ≤ 50 m thick debris 2a). The region generally consists of dissected basin cover [24-26]. Global circulation models have also massifs and expansive nearly flat-lying plains. Massif shown that regions of net ice accumulation are in blocks vary in size and relief with the largest up to 80 agreement with some locations of LDAs [27,28]. km across (long dimension) and 3.5 km high. The Recent mapping of LDAs and other ice-rich flow plains, generally between -1000 to -1500 m, have features in the southern hemisphere of Mars, hereafter slopes of less than one degree in the middle to lower referred to collectively as ice-rich debris aprons (IDA), portions of the basin. suggests that these are more widespread than In eastern Nereidum Montes, CTX images of the previously noted and extend further east of Hellas [29]. sheet-like lobate features show evidence for past With the availability of global 100 m/pixel THEMIS glaciation. Figure 2b shows an area where possible day IR data, this mapping has now expanded to the valley glaciers in the upper reaches of Nereidum entire hemisphere between 30° and 50° S. IDA margins Montes once flowed into open plains and may have were mapped at ~1:180,000-1:250,000 scale and only later retreated. The glacially-modified surfaces and features larger than 1 km2 were considered. In our adjacent massifs have various textures ranging from mapping, an IDA can be a single lobate feature, a smooth to pitted, similar to those on eastern Hellas collection of coalesced lobate features that form one IDAs. Downslope from the glacial flows, the terrain larger feature, or many separate lobate features that exhibits features analogous to those seen in terrestrial share a common source. IDAs generally extend glacial outwash plains, including large boulders, downslope from high relief source regions such as meltwater channels, a streamlined island and an outlet impact crater walls (interior and exterior), massifs valley (Fig. 2c). In addition, massif LDAs, arcuate (single and multiple), valley walls, and escarpments. ridges, viscous flow features, and spatulate protalus Mapping Results: A total of 3,365 IDA features lobes common to eastern Hellas are also observed [31]. were mapped using ArcGIS software, covering Implications and Future Work: Eastern 379,569 km2 of the study area (1.4%). Figure 1 shows Nereidum Montes is a region that appears to have the percent surface area covered by IDAs in 2° latitude experienced significant past glacial activity from our and 2° longitude bins. Regions with the highest initial observations. Further analyses of ice-rich percentages (i.e. > 10%) occur in eastern Hellas and features using CTX will provide additional information within Nereidum Montes in northeast Argyre basin on potential glacial activity and allow for detailed (314°-334° E). The eastern Hellas region extends from comparisons between ice-driven modification of the the distal end of across Promethei Terra and terrains adjacent to the Argyre and Hellas impact into the southern highlands (80°-150° E). Other regions basins. with smaller concentrations at lower percentages References: [1] Sharp (1973) JGR, 78, 4073-4083. [2] include Terra Sirenum, northwest Argyre, western Carr and Schaber (1977) JGR, 82, 4039-4054. [3] Squyres Hellas, and central Hellas. (1978) Icarus, 34, 600-613. [4] Squyres (1979) JGR, 84, 45th Lunar and Planetary Science Conference (2014) 2066.pdf

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Figure 1. Map of percent surface area coverage by IDAs in 2° longitude by 2° latitude bins across the southern hemisphere. Regions with the highest percentages (> 10%) are where the greatest concentrations exist (eastern Hellas and northeast Argyre; white ovals). Other minor concentrations at lower percentages include Terra Sirenum, northwest Argyre, western Hellas, and central Hellas (black ovals).

Figure 2. (A) View of Argyre impact basin and Nereidum Montes. Background THEMIS day IR base mosaic and MOLA topography (hot colors=higher elevations). (B) Portion of Nereidum Montes in CTX where possible valley glaciers once flowed into open plains. Subsequent glacial retreat and deposition of ice-rich mantle materials (smooth to pitted textures) may have occurred. (C) Terrain below the smooth plains with analogous features seen in terrestrial glacial outwash plains such as large boulders and meltwater channels (portion of CTX P14_006466_1930).